U.S. patent application number 11/272183 was filed with the patent office on 2006-10-05 for vertical production of photovoltaic devices.
This patent application is currently assigned to Daystar Technologies, Inc.. Invention is credited to John R. Tuttle.
Application Number | 20060219547 11/272183 |
Document ID | / |
Family ID | 36337254 |
Filed Date | 2006-10-05 |
United States Patent
Application |
20060219547 |
Kind Code |
A1 |
Tuttle; John R. |
October 5, 2006 |
Vertical production of photovoltaic devices
Abstract
The present invention provides a photovoltaic thin-film solar
cell produced by a providing a vertically oriented pallet based
substrate to a series of reaction chambers where layers can be
sequentially formed on the pallet.
Inventors: |
Tuttle; John R.;
(Mechanicville, NY) |
Correspondence
Address: |
HISCOCK & BARCLAY, LLP
2000 HSBC PLAZA
ROCHESTER
NY
14604-2404
US
|
Assignee: |
Daystar Technologies, Inc.
|
Family ID: |
36337254 |
Appl. No.: |
11/272183 |
Filed: |
November 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60626843 |
Nov 10, 2004 |
|
|
|
Current U.S.
Class: |
204/192.12 ;
257/E31.027 |
Current CPC
Class: |
H01L 31/0322 20130101;
Y02P 70/50 20151101; H01L 31/072 20130101; H01L 21/67155 20130101;
Y02E 10/541 20130101; H01L 31/1876 20130101; Y02P 70/521
20151101 |
Class at
Publication: |
204/192.12 |
International
Class: |
C23C 14/00 20060101
C23C014/00 |
Claims
1. An apparatus for manufacturing a photovoltaic device comprising
a means for providing a vertically oriented substrate to a first
reaction zone; a plurality of reaction zones including at least a
zone capable of providing an environment for deposition of a back
contact layer; a zone capable of providing an environment for
depositing a p-type semiconductor layer; and a zone capable of
providing an environment for depositing a n-type semiconductor
layer.
2. The apparatus of claim 1 wherein said means for providing a
vertically oriented substrate is a pallet based system and means
for transporting pallets through the plurality of reaction
zones.
3. The apparatus of claim 1 which further comprises a second means
for transporting a vertically oriented substrate to said plurality
of reaction zones.
4. A method for manufacturing a photovoltaic device comprising
providing a means capable of vertically holding a substrate, in
sequence to a plurality of reactor zones wherein said plurality of
zones includes at least one zone depositing a p-type semiconductor
layer.
5. A method for manufacturing a photovoltaic cell comprising: a.
providing a plurality of vertically disposed substrates; b.
depositing a conductive film on the surface of said plurality of
substrates; c. wherein the conductive film includes a plurality of
discrete layers of conductive materials; and d. depositing an
n-type semiconductor layer on an p-type absorber layer forming a
p-n junction.
6. The method of claim 5 further depositing at least one p-type
semiconductor layer on the conductive film, wherein the p-type
semiconductor layer includes a copper indium di-selenide based
alloy material.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Patent Application Ser. No. 60/626,843, filed Nov. 10, 2004.
FIELD OF THE INVENTION
[0002] The invention disclosed herein relates generally to the
manufacture of photovoltaic devices and more specifically to an
apparatus for manufacturing thin film the product and method of
manufacturing thin-film solar cells using a vertically oriented
pallet based system.
BACKGROUND OF THE INVENTION
[0003] The benefits of renewable energy are not fully reflected in
the market price. While alternative energy sources such as
photovoltaic (PV) cells offer clean, reliable, and renewable
energy, high product costs and lack of production reliability have
kept these devices from being a viable commercial product. With the
demand for energy going up, the world demand for alternatives to
present energy sources is increasing.
[0004] Although relatively efficient thin-film PV cells can be
manufactured in the laboratory, it has proven difficult to
commercially scale manufacturing processes with the consistent
repeatability and efficiency critical for commercial viability.
Moreover, the cost associated with manufacturing is an important
factor preventing the broader commercialization of thin-film solar
cells. The lack of an efficient thin-film manufacturing process has
contributed to the failure of PV cells to effectively replace
alternate energy sources in the market.
[0005] Thin-film PV cells can be manufactured according to varied
designs. In a thin-film PV cell, a thin semiconductor layer of PV
materials is deposited on a supporting layer such as glass, metal,
or plastic foil. Since thin-film materials have higher light
absorptivity than crystalline materials, PV materials are deposited
in extremely thin consecutive layers of atoms, molecules, or ions.
The typical active area of thin-film PV cells is only a few
micrometers thick. The basic photovoltaic stack design exemplifies
the typical structure of a PV cell. In that design, the thin-film
solar cell comprises a substrate, a barrier layer, a back contact
layer, a p-type absorber layer, an n-type junction buffer layer, an
intrinsic transparent oxide layer, and a transparent conducting
oxide layer. Compounds of copper indium gallium diselenide (CIGS)
have the most promise for use in absorber layers in thin-film cells
and fit within the classification of copper-indium selenium class,
called CIS materials. CIGS films are typically deposited by
vacuum-based techniques.
[0006] Thin-film manufacturing processes suffer from low yield due
to defects in the product that occur during the course of
deposition. Specifically, these defects are caused by contamination
occurring during processing and materials handling, and the
breakage of glass, metal, or plastic substrates. Thus, a process
for manufacturing thin-film solar cells that both limits potential
contamination during processing and concurrently minimizes
substrate breakage is desired in the art.
[0007] Currently, cells are manufactured using a multi-step batch
process wherein each product piece is transferred between reaction
steps. This transfer is bulky and requires the reaction in chambers
to be cycled. A typical process consists of a series of individual
batch processing chambers, each specifically designed for the
formation of various layers in the cell. Problematically, the
substrate is transferred from vacuum to air--and back
again--several times. Such vacuum breaks may result in
contamination of the product. Thus, a process that minimizes vacuum
breaks is desired in the art.
[0008] While an alternate system uses a series of individual batch
processing chambers coupled with a roll-to-roll continuous process
for each chamber, the discontinuity of the system and the need to
break vacuum continue to be major drawbacks. Additionally, the
roll-to-roll process may impose flexing stress on a glass or metal
substrate, resulting in fracturing and breakage. Such defects
compromise layer cohesiveness and may result in a zero yield.
[0009] Also contributing to the low yield in PV cell manufacturing
is the requirement of high-temperature deposition processes. High
temperatures are generally incompatible with all presently known
flexible polyimide or other polymer substrate materials.
[0010] For example, U.S. Patent Application 2004/0063320, published
by Hollars on Apr. 1, 2004, discloses a general methodology for
continuously producing photovoltaic stacks using a roll-to-roll
system. As discussed above, this process requires the application
of flexing stress to the substrate. This stress potentially results
in fractures and breakage where the substrate material is glass or
metal. Fractures or breakage reduce high quality stack structures
and lower manufacturing yield. Thus, to be a commercially viable
process, the disclosed system requires a flexible substrate for the
production of the stack. However, no currently known flexible
polymer materials can withstand the high-temperature deposition
process.
[0011] Furthermore, Hollars does not teach any specific apparatus
for optimizing the product flow through their continuous system.
Horizontal processing is still used as the basic deposition and
reaction orientation of the pieces being worked on, and do not
employ any scheme for passing multiple processing streams through
each or any of the zones.
[0012] Therefore, a process that does not impose flexing stress on
the substrates, where the substrates can withstand the
high-temperature deposition process, is desired in the art. So a
process for manufacturing PV work pieces effectively, and capable
of large scale production are needed.
SUMMARY OF THE INVENTION
[0013] The present invention provides a photovoltaic produced by
providing a vertically oriented product substrate is provided by a
continuous backing, a conveyor belts means or by a pallet-based
transport means to a series of reaction chambers where sequentially
a barrier layer, a back contact layer, an p-type semiconductor
layer, alkali materials, an n-type junction buffer layer, an
intrinsic transparent oxide layer, a transparent conducting oxide
layer and a top metal grid can be formed on the pallet.
[0014] A method is further disclosed for forming a photovoltaic
device by employing a train of the pallet based holders loaded with
work pieces in a vertical orientation and with work piece
substrates provided on both the front and the back of each of the
pallets so that the controlled reaction chambers produces roughly
double the amount of product a single sided pallet would. In this
embodiment, a series of pallets are passed at a defined rate
through a reactor having a plurality of processing zones, wherein
each zone is dedicated to one production step stage of device
manufacture.
[0015] The specific production steps production that this
vertically oriented product train would be processed through might
include: a load or isolation zone for substrate preparation;
environments for depositing a barrier layer, a back contact layer,
a semiconductor layer or layers, and alkali materials; an
environment for the thermal treatment of one or more of the
previous layers; and an environment for the deposition of: an
n-type compound semi-conductor wherein this layer serves as a
junction buffer layer, an intrinsic transparent oxide layer, and a
conducting transparent oxide layer. In a further embodiment, the
process may be adjusted to comprise greater fewer zones in order to
fabricate a thin film solar cell having more or fewer layers.
[0016] A vertically-oriented pallet type system may be employed
where a plurality of work pieces are held as a pallet and a
plurality of pallets are processed though a continuous reactor step
apparatus. This pallet based system allows continuous processing of
smaller work pieces and alternative materials handling steps, such
as pallet stacking in intermediate or final steps.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows an embodiment of a thin-film solar cell
produced by the production technology of the present invention.
[0018] FIG. 2 schematically represents a reactor for forming solar
cells.
[0019] FIG. 3 shows a plurality of work piece substrates on a
device capable of affixing the substrates onto a carrier, that also
has means that allow the pieces to be advanced in a precise fashion
through the production apparatus.
[0020] FIG. 4 shows a schematic of the pallet used in the present
invention populated with a plurality of substrate work pieces.
[0021] FIG. 5A shows an embodiment of the processing method wherein
two substrates are fed and processed simultaneously by a sequential
sputter-evaporate process in accordance with the present
invention.
[0022] FIG. 5B shows a top view of an embodiment of the processing
method wherein two substrates are fed and processed simultaneously
by a sequential sputter-evaporate/sputter-evaporate process.
[0023] FIG. 6 illustrates another embodiment of the process in
accordance with the invention wherein zones further comprise one or
more sub-zones.
DETAILED DESCRIPTION OF THE INVENTION
General Photovoltaic Stack Designs
[0024] The present invention employs a new production apparatus to
produce photovoltaic devices. Of course, the particular apparatus
will depend upon the specific photovoltaic device design, which can
be varied.
[0025] Viewing FIG. 1, all layers are deposited on a substrate 105
which may comprise one of a plurality of functional materials, for
example, glass, metal, ceramic, or plastic. Deposited directly on
the substrate 105 is a barrier layer 110. The barrier layer 110
comprises a thin conductor or very thin insulating material and
serves to block the out diffusion of undesirable elements or
compounds from the substrate to the rest of the cell. This barrier
layer 110 may comprise chromium, titanium, silicon oxide, titanium
nitride and related materials that have the requisite conductivity
and durability. The next deposited layer is the back contact layer
120 comprising non-reactive metals such as molybdenum. The next
layer is deposited upon the back contact layer 120 and is a p-type
semiconductor layer 130 to improve adhesion between an absorber
layer 155 and the back contact 120. The p-type semiconductor layer
130 may be a I-III.sub.a,b-VI isotype semiconductor, but the
preferred composition is Cu:Ga:Se; Cu:Al:Se or Cu:In:Se alloyed
with either of the previous compounds.
[0026] In this embodiment, the formation of a p-type absorber layer
involves the interdiffusion of a number of discrete layers.
Ultimately, as seen in FIG. 1, the the p-type semiconductor layers
130 and 150 combine into a single composite layer 155 which serves
as the prime absorber of solar energy. In this embodiment, alkali
materials 140 are added for the purpose of seeding the growth of
subsequent layers as well as increasing the carrier concentration
and grain size of the absorber layer 155, thereby increasing the
conversion efficiency of the solar cell. Once deposited, the layers
are thermally treated at a temperature of about 400.degree.
C.-600.degree. C.
[0027] After the thermal treatment, the photovoltaic production
process is continued by the deposition of an n-type junction buffer
layer 160. This layer 160 will ultimately interact with the
absorber layer 155 to form the necessary p-n junction 165. A
transparent intrinsic oxide layer 170 is deposited next to serve as
a hetero-junction with the CIGS absorber. Finally, a conducting
transparent oxide layer 180 is deposited to function as the top of
the electrode of the cell. This final layer is conductive and may
carry current to a grid carrier that allows the current generated
to be carried away.
General Apparatus Configurations
[0028] A first embodiment of the invention is an apparatus for
manufacturing a photovoltaic device comprising a means for
providing a means for presenting the work pieces to the production
apparatus where the orientation of the work pieces is vertical.
This vertical orientation of the production train allows the work
pieces to be disposed on the front and back of the product train
and allows an increase in the capacity of the manufacturing
apparatus. Surprisingly it has been found that provided the work
piece substrates on a vertical axis can be accomplished by
employing several factors which include: [0029] Limited substrate
height so that reaction chamber technology can be optimized [0030]
Adequately isolation of each deposition or reaction chamber from
the next [0031] Adequate monitoring and control of the reaction
materials and deposition sources [0032] Precise temperature
control
[0033] It has been found, however, that a system needs a vertical
substrate which may employ the positioning of target substrates on
both sides of the vertical plane so that a two fold instance in
production can be achieved and better and more economical use of
the reaction parameters which are so assiduously controlled which
involve relatively low pressures and higher temperatures can be
more economically achieved.
[0034] A plurality of pallets holding multiple substrate pieces may
be employed as the means for holding the substrates as the
production train, in sequence, is transported through the plurality
of reaction zones. These reaction zones include at least a zone
capable of providing an environment for deposition of a
semiconductor layer, and a zone capable of providing an environment
for depositing precursor materials to form a p-type absorber
layer.
[0035] FIG. 4 shows a schematic view of a pallet. The pallet
provides a holding basis 400 for a plurality of small PV workpiece
substrates 410, or working substrates fixedly attached to the
pallet in a pre-determined manner so that the individual work
pieces are presented in each treatment chamber in a precise and
controllable fashion. The pallet itself is engineered so that the
position of the pallet can be precisely determined. The pallet also
has a means 420 for allowing attachment to a drive means to advance
the pallet through the treatment chamber. Materials of the body of
the pallet are chosen so that they are thermally stable and do not
interact with the treatment or deposition materials used in the
reaction or deposition chamber.
[0036] Furthermore, the means for securing the work pieces to the
pallet are releasable. In some instances the means for affixing the
work piece is magnetic, either because the substrate of the
workpiece is itself ferro-magnetic, or with an overlay that hold
the individual pieces to the body of the pallet.
[0037] In a preferred embodiment, the process may further comprise
a substrate that runs back-to-back with the substrate. In this
embodiment substrates and are oriented vertically in a back-to-back
configuration and run through zones performing identical process
operations.
[0038] FIG. 5A shows a top illustration of a portion of a reactor
500 processing substrates 501 and 502 in a back-to-back fashion and
also illustrates a sequential sputter-evaporate process isolated by
zone 511. To achieve back-to-back processing, heat sources 503 for
substrate 501 are mirrored as heat sources 507 for substrate 502.
Likewise, sputtering source 504, heat sources 505, and evaporative
sources 506 for substrate 501 are mirrored for substrate 502 as
sputtering source 508, heat sources 509, and evaporative sources
510. FIG. 5A shows this vertical two sided manufacturing process at
the top where the two substrates in which the photo devices are
being made. Substrates 501 and 502 are processed from left to right
through the heating, sputtering and evaporation chambers of a
device forming layers to thin films of the PV device. The substrate
is passed by sequential heaters 503 and 507 then exposed to
sputtering target 503 and 509 with an atmosphere of 1e-3-1e-2 torr.
The substrates are then transported through differential pumping
chamber at 1e-7-1e-6 torr and then presented to an evaporation
deposition chamber where heaters 505 and 509 are used to heat each
of the respective substrate 501 and 502 and evaporation sources of
gases are provided 506 and 510 respectively.
[0039] FIG. 5B shows a top illustration of a portion of a reactor
512 processing substrates 521 and 522 in a back-to-back fashion
with a sequential sputter-evaporate/sputter-evaporate process. As
in FIG. 5A, sputter sources 534 for substrate 521 are mirrored as
sputter sources 528 for substrate 522. Likewise, heat sources 523
and 526, evaporative sources 524 and 527, and sputtering source 525
for substrate 521 are mirrored for substrate 522 as heat sources
529 and 532, evaporative sources 530 and 533, and sputtering source
531. Hence, with the simple duplication of heat and material
sources, solar cell production may be effectively doubled within
the same machine.
Alternative Pallet Based Manufacturing Schemes
[0040] FIG. 2 schematically represents a reactor 200 for forming
solar cells. A substrate 205 is fed left to right through the
reactor. The reactor 200 includes one or more processing zones,
referred to in FIG. 2 as 220, 230, 240 and 250, wherein each
processing zone comprises an environment for depositing materials
on a substrate 205. The zones are mechanically or operatively
linked together within the reactor 200. As used herein, the term
environment refers to a profile of conditions for depositing or
reacting a material layer or mixture of materials on the substrate
205 while the substrate 205 is in a particular zone.
[0041] Each zone is configured according to which layer of the
solar cell is being processed. For example, a zone may be
configured to perform a sputtering operation, including heat
sources and one or more source targets.
[0042] Preferably, an elongated substrate 205 is passed through the
various processing zones at a controllable rate. It is further
contemplated that the substrate 205 may have a translational speed
of 0.5 m/min to about 2 m/min. Accordingly, the process internal to
each of the zones is preferably tuned to form the desired
cross-section given the residence time the material is proximate to
a particular source material, given the desired transport speed.
Thus, the characteristics of each process, such as material and
process choice, temperature, pressure, or sputtering delivery rate,
etc., may be chosen to insure that constituent materials are
properly delivered given the stack's residence time as determined
by the transport or translation speed.
[0043] According to the invention, the substrate 205 may be
transported through the process in a vertically oriented palletized
fashion in a "picture frame" type mount for indexing and
transportation through the process, the latter of which is
illustrated in FIG. 3. Referring to FIG. 3 one substrate or group
of substrates 310 are mounted on a pallet 320 that translates
through one or more zones 330 and 340 on track 350. In alternate
embodiments the process may further comprise a second substrate or
set of substrates placed in a back to back configuration with
substrate 310.
[0044] It is contemplated that the background pressure within the
various zones will range from 10.sup.-6 torr to 10.sup.-3 torr.
Pressures above base-vacuum (10.sup.-6 torr) may be achieved by the
addition of a pure gas such as Argon, Nitrogen or Oxygen.
Preferably, the rate R is constant resulting in the substrate 205
passing through the reactor 200 from entrance 201 to exit 202
without stopping. It will be appreciated by those of ordinary skill
in the art that a solar cell stack may thus be formed in a
continuous fashion on the substrate 205, without the need for the
substrate 205 to ever stop within the reactor 200.
[0045] The reactor in FIG. 2 may further comprise vacuum isolation
sub-zones or slit valves configured to isolate adjacent process
zones. The vacuum isolation sub-zones or slit valves are provided
to facilitate the continuous transport of the substrate between
different pressure environments.
[0046] The reactor shown in FIG. 2 is a plurality of N-processing
zones 220, 230, 240 and 250. However, it should be recognized by
one skilled in the art that the reactor may comprise zones 220,
230, 240, 250 . . . N zones. The load/unload zones 210/211 comprise
zones that can be isolated from the rest of the reactor and can be
open to atmosphere.
[0047] In a preferred embodiment, the process may further comprise
a substrate 206 that runs back-to-back with substrate 205. In this
embodiment substrates 206 and 205 are oriented vertically in a
back-to-back configuration and run through zones 220, 230, 240, and
250 performing identical process operations 222/221, 232/231,
242/241 and 252/251.
SPECIFIC PROCESSING STEPS
[0048] Of course, the method steps for producing a particular PV
article depends upon the specific design of that article. CIS based
PVs will have a different production method than Si based systems.
The present invention is not so limited to one PV type and in
general any PV could be made with the technology of the
invention.
[0049] In cases of CIGS, the specific steps might include: loading
a substrate through an isolated loading zone or like unit 210. In
various embodiments, the isolation zone 210 is contained within the
reactor 200. Alternatively, the isolation zone 210 may be attached
to the outer portion of the reactor 200. The first processing zone
210 may further comprise a substrate preparation environment to
remove any residual imperfections at the atomic level of the
surface. The substrate preparation may include: ion beam,
deposition, heating, or sputter-etch. These methods are known in
the art and will not be discussed further.
[0050] A second processing zone may be environment for depositing a
barrier layer for substrate impurity isolation, wherein the barrier
layer provides an electrically conductive path between the
substrate and subsequent layers. In a preferred embodiment, the
barrier layer comprises an element such as chromium or titanium
delivered by a sputtering process. Preferably, the environment
comprises a pressure in the range of about 10.sup.-3 torr to about
10.sup.-2 torr at ambient temperature.
[0051] A third processing zone downstream from the previous zones
comprises an environment for the deposition of a metallic layer to
serve as a back contact layer. The back contact layer comprises a
thickness that provides a conductive path for electrical current.
In addition, the back contact layer serves as the first conducting
layer of the solar cell stack. The layer may further serve to
prevent the diffusion of chemical compounds such as impurities from
the substrate to the remainder of the solar cell structure or as a
thermal expansion buffer between the substrate layer and the
remainder of the solar cell structure. Preferably, the back contact
layer comprises molybdenum, however, the back contact layer may
comprise other conductive metals such as aluminum, copper or
silver.
[0052] A fourth zone provides an environment for deposition of a
p-type semiconductor layer. As used herein, the p-type
semiconductor layer may serve as an epitaxial template for absorber
growth. Preferably, the p-type semiconductor layer is an isotype
I-IIIVI.sub.2 material, wherein the optical band gap of this
material is higher than the average optical band gap of the p-type
absorber layer. For example, a semiconductor layer may comprise
Cu:Ga:Se; Cu:AI:Se or alloys of Cu:In:Se with either of the
previous compounds. Preferably, the materials are delivered by a
sputtering process at a background pressure of 10.sup.-6 to
10.sup.-2 torr and at temperatures ranging from ambient up to about
300.degree. C. Preferably, temperatures range from ambient to about
200.degree. C.
[0053] A fifth zone, downstream from the previous zones, provides
an environment for the deposition of alkali materials to enhance
the growth and the electrical performance of a p-type absorber.
Preferably, the alkali materials are sputtered, at ambient
temperature and a pressure range of about 10.sup.-6 torr to
10.sup.-2 torr. Preferably, the material comprises NaF, Na.sub.2Se,
Na.sub.2S or KCl or like compounds wherein the thickness ranges
from about 150 nm to about 500 nm.
[0054] A sixth zone, also downstream from the previous zones, may
comprise an environment for the deposition of additional
semiconductor layers comprising precursor materials for the p-type
absorber layer. In a preferred embodiment, the sixth zone may
further comprise one or more sub-zones for the deposition of the
precursor layers. In one embodiment, the layer is formed by first
delivering precursor materials in one or more contiguous sub-zones,
then reacting the precursor materials into the final p-type
absorber in a downstream thermal treatment zone. Thus, especially
for CIGS Systems, there may be two material deposition steps and a
third thermal treatment step in the format of the layer.
[0055] In the precursor delivery zones, the layer of precursor
materials is deposited in a wide variety of ways, including
evaporation, sputtering, and chemical vapor deposition or
combinations thereof Preferably, the precursor material may be
delivered at temperatures ranging from about 200.degree.
C.-300.degree. C. It is desired that the precursor materials react
to form the final p-type absorber as rapidly as possible. As
previously discussed, to this end, the precursor layer or layers
may be formed as a mixture or a series of thin layers.
[0056] A manufacturing device may also have seventh processing zone
downstream from previous processing zones for the thermal treatment
of one or more of the previous layers. The term multinaries
includes binaries, ternaries, and the like. Preferably, thermal
treatment reacts previously unreacted elements or multinaries. For
example, in one embodiment it is preferred to have Cu, In, Se, and
Ga in various combinations and ratios of multinary compounds of
elements as the source for deposition on the work piece. The
reactive environment includes selenium and sulfur in varying
proportions and ranges in temperature from about 400.degree. C. to
about 600.degree. C. with or without a background inert gas
environment. In various embodiments, processing time may be
minimized to one minute or less by optimizing mixing of the
precursors. Optimal pressures within the environment depend on
whether the environment is reactive or inert. According to the
invention, within the thermal treatment zone, the pressures range
from about 10.sup.-6 to about 10.sup.-2 torr. However, it should be
noted that these ranges depend very much on the reactor design for
the stage, the designer of the photovoltaic device and the
operational variables of the apparatus as a whole.
[0057] The reactor may have an eighth processing zone for the
formation of an n-type semiconductor layer or junction partner. The
junction layer is selected from the family II-VI, or IIIx VI. For
example, the junction layer may comprise ZnO, ZnSe, ZnS, In, Se or
In.sub.NS deposited by evaporation, sublimation or chemical vapor
deposition methodologies. The temperatures range from about
200.degree. C. to about 400.degree. C.
[0058] Additionally, the process may also have a ninth zone having
an environment for deposition of an intrinsic layer of a
transparent oxide, for example ZnO. According to the invention, the
intrinsic transparent oxide layer may be deposited by a variety of
methods including for example, RF sputtering, CVD or MOCVD.
[0059] In various embodiments, the process further has a tenth zone
with an environment for the deposition of a transparent conductive
oxide layer to serve as the top electrode for the solar cell. In
one embodiment for example, aluminum doped ZnO is sputter
deposited. Preferably, the environment comprises a temperature of
about 200.degree. C. and a pressure of about 5 millitorr.
Alternatively, ITO (Indium Tin Oxide) or similar may be used.
[0060] In one embodiment, as described above, the reactor may
comprise discrete zones wherein each zone corresponds to one layer
of photovoltaic device formation. In a preferred embodiment
however, zones comprising similar constituents and or environment
conditions may be combined thereby reducing the total number of
zones in the reactor.
[0061] For example, in FIG. 6, zone 610 comprises sub-zones 611 and
612, zone 615 comprises sub-zones 616 and 617, and zone 620
comprises one zone, wherein each zone and sub-zone comprises a
predetermined environment. In this example, a material A may be
deposited in sub-zone 611 and a different material B may be
deposited in sub-zone 612, wherein the environment of sub-zone 612
downstream from material A differs from the environment in sub-zone
611. Thus, the substrate 605 may be subjected to a different
temperature or other process profiles while in different regions of
the same zone 610. According to this embodiment, the zone may be
defined as having a predetermined pressure, and a zone may include
one or more regions, sub-zones, or phases therein, with each
sub-zone configured to deposit or react a desired material or
materials within the same pressure environment.
[0062] The substrate 605 may then be passed to chamber 615, where
material C is deposited within sub-zone 616, and material D is
deposited in sub-zone 617. Finally, the substrate 605 reaches a
zone 620, where a single material E is deposited.
[0063] As will be appreciated by those of ordinary skill in. the
art, the reactor 600 may be described as having a series of zones
disposed between the entrance and exit of the reactor along a path
defined by the translation of the substrate. Within each zone, one
or more constituent environments or sub-zones may be provided to
deposit or react a selected target material or materials, resulting
in a continuous process for forming a solar cell stack. Once the
substrate enters the reactor, the various layers of a solar stack
are deposited and formed in a sequential fashion, with each
downstream process in succession contributing to the formation of
the solar cell stack until a finished thin film solar cell is
presented at the exit of the reactor.
[0064] While the present technique has been couched in terms of
CIGS based photovoltaic stack designs it must be understood that
the technique may also be employed for the production of other
photovoltaic designs including production of silicon based systems
such as those discussed in state of the art. For instance, it would
be possible to use to include carbon or germanium atoms in
hydrogenated amorphous silicon alloys in order to adjust their
optical bandgap. For example, carbon has a larger bandgap than
silicon and thus inclusion of carbon in a hydrogenated amorphous
silicon alloy increases the alloy's bandgap. Conversely, germanium
has a smaller bandgap than silicon and thus inclusion of germanium
in a hydrogenated amorphous silicon alloy decreases the alloy's
bandgap.
[0065] Similarly one could incorporate boron or phosphorus atoms in
hydrogenated amorphous silicon alloys in order to adjust their
conductive properties. Including boron in a hydrogenated amorphous
silicon alloy creates a positively doped conductive region.
Conversely, including phosphorus in a hydrogenated amorphous
silicon alloy creates a negatively doped conductive region.
[0066] Hydrogenated amorphous silicon alloy films are prepared by
deposition in a deposition chamber. Heretofore, in preparing
hydrogenated amorphous silicon alloys by deposition in a deposition
chamber, carbon, germanium, boron or phosphorus have been
incorporated into the alloys by including in the deposition gas
mixture carbon, germanium, boron or phosphorus containing gases
such as methane (CH.sub.4), germane (GeH.sub.4), germanium
tetrafluoride (GeF.sub.4), higher order germanes such as digermane
(Ge.sub.2 H.sub.6), diborane (B.sub.2 H.sub.6) or phosphine
(PH.sub.3). See for example, U.S. Pat. Nos. 4,491,626, 4,142,195,
4,363,828, 4,504,518, 4,344,984, 4,435,445, and 4,394,400. A
drawback of this practice, however, is that the way in which the
carbon, germanium, boron or phosphorus atoms are incorporated into
the hydrogenated amorphous silicon alloy is not controlled. That
is, these elements are incorporated into the resulting alloy in a
highly random manner thereby increasing the likelihood of
undesirable chemical bonds.
[0067] Thus, in cases where PV devices are manufactured, and
specific and controlled reaction and or deposition conditions are
required to produce the films of the PV, the present invention
technology will be useful.
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